Over the past decade, the promulgation of new more stringent ambient standards has led to a different thought process on previous conservative modeling assumptions and approaches. For example, we can no longer: 1. Assume 100% conversion of NO_X to NO_2. Consideration needs to be given to the right approach for modeling NO_2 for each application. That could be simply using ARM2 or could extend to the use of PVMRM or OLM with refined in-stack NO_2/NO_X ratios and consideration of near-field ozone reaction rates. 2. Assume that the ambient background is a fixed value. When assessing NAAQS compliance, consideration must be given to variability in ambient background relative to season and hour of day. 3. Assume the use of permit allowable emission rates for all sources in a model application is appropriate. Consideration must be given to the actual operating level of nearby sources (including its intermittent nature) to provide a more realistic estimate of emissions for modeling. Consideration must also be given to the impact nearby sources have on the ambient background used for the NAAQS demonstrations to avoid double counting. In some cases, the monitored background already includes the impacts of nearby sources. Progress has been encouraging, and we need to continue to promote innovation in assumptions and techniques.
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